Power-on reset circuits for providing a power-on reset signal have become necessary in some applications where certain circuit elements must begin operation in synchronization as power is applied. The power-on reset signal is held at a first level until the applied power reaches a predetermined voltage level at which time the power-on reset signal is brought to a second level. Information is thereby available to other circuitry that the predetermined voltage level has been reached and that there is then sufficient voltage for operation to begin.
Because some circuits require a certain amount of time to reach proper operating condition after the predetermined voltage is reached in a case where the predetermined voltage is reached very quickly, the power-on reset signal must remain at the first level for a minimum time duration which is sufficient to enable all relevant circuitry to reach proper operating condition.
In the prior art, the power-on reset circuits which are used on integrated circuits require inordinate amounts of chip area, partially due to the use of resistors. Resistors on integrated circuits require substantially more chip area than do transistors or capacitors. One resistor is usually required for the portion of the circuit which ensures that the power-on reset signal does not reach the second level until the power reaches the predetermined voltage level, and another resistor is required for the portion of the circuit which ensures that the power-on reset signal is held at the first level for at least the time duration which is sufficient to enable all relevant circuitry to reach proper operating condition.
An object of the invention is to provide a power-on reset circuit which requires less chip area of an integrated circuit to fabricate.
A further object of the invention is to eliminate the use of a resistor in the portion of the circuit which ensures that the power-on reset signal does not reach the second level until the power reaches the predetermined voltage level.
A still further object of the invention is to eliminate the use of a resistor in the portion of the circuit which ensures that the power-on reset signal is held at the first level for at least the time duration which is sufficient to enable all relevant circuitry to reach proper operating condition.